JPS62152370A - Dc-dc converter of self-excited type - Google Patents

Abstract

PURPOSE:To stabilize output voltage, by adding fly-back voltage with output voltage stored in a transformer, while a transistor is OFF, and by ON/OFF- controlling the transistor according to the result of comparing the added voltage with reference voltage. CONSTITUTION:By a diode D2 connected to the primary winding NB of a transformer T1, negative potential is taken out and is smoothed by a capacitor C3, and negative constant voltage is obtained by a resistance R5 and a Zener diode ZD2. Between the negative potential and output voltage, detection resistances R3, R4 are connected, and the divided voltage is compared with the Zener voltage of a Zener diode ZD1, and a control transistor Tr2 is controlled on the result of the comparison.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a self-excited DC/DC converter, and particularly to a DC/DC converter capable of obtaining a stable DC voltage of low voltage 1, for example, 5V or 3V.

[Conventional technology]

FIG. 4 shows a basic diagram of a self-excited DC/DC converter.

One primary winding NP of the transformer T and the collector emitter of the transistor Tr1 are connected in series with respect to the DC input voltage vin. A starting resistor R□ is connected between the positive terminal of the DC input and the pace terminal of the transistor Tr1. When the pace current of the transistor Tr1 flows from the DC input through the entire starting resistor R1, a collector current flows, and a current flows to the -order winding Np. Then, transformer T0
is excited and a voltage is induced in the other primary winding NB.

As a result, when the base current of the transistor Tr1 increases, the collector current further increases, and the current in the -order winding NP also increases. Such positive feedback is limited by the resistor R2 and capacitor C1 connected between the negative winding N and the base of the transistor Tr1.

This operation continues until the collector current of the transistor Tr1 increases to a value corresponding to the product of its base current and the DC amplification factor, and when this value is exceeded, the transistor Tr1 is abruptly turned off. Then, the magnetic energy stored in the transformer T1 is outputted from the secondary winding NL, and the half-wave component passed through the diode D1 is transferred to the capacitor C.
DC output V is smoothed by 2. It becomes ut.

After the magnetic energy of transformer T1 is released.

The transistor Tr1 is turned on again by positive feedback as described above.

FIG. 5 shows a conventional example in which the basic circuit shown in FIG. 4 is improved. This circuit has an output voltage V as shown in FIG. In order to control ut to a constant voltage, the transistor Tr2 controls the on-time of the transistor Tr1 to control the magnetic energy stored in the transformer. That is, the detection resistors R3, R,
The output voltage vout is divided and compared with the potential of the Zener diode ZD. When the 1 partial voltage becomes higher than the Zener potential of the Zener diode ZD1, a current flows through the Zener diode ZD1, turning on the control transistor Tr2 and turning off the switching transistor Tr1.

Conversely, the DC output V. When ut is low, the partial voltage is also low by 1 and the transistor Tr2 is off, so the transistor Tr1 is on for a long time and the transformer T.

A large amount of magnetic energy is accumulated in the magnetic field, and the output voltage becomes high.

Here, the DC output V. The Zener diode ZD1 used when ut is required at low voltage is shown in curve 31 in Figure 6.
As shown in the figure, as the current I flowing through the Zener diode increases, the Zener voltage Vf tends to gradually increase.

On the other hand, in a Zener diode with a large Zener voltage vf, as shown by curve 32 in FIG. 6, the change in voltage f due to an increase in Zener current is very small. Therefore, when obtaining a low voltage, in the case of the circuit shown in FIG.
When n is high and the four-way current of the transistor Tr1 increases, it is necessary to increase the current flowing through the transistor Tr2 to turn off the transistor Tr1. In this case, the Zener voltage of the Zener diode ZD1 increases, resulting in an increase in the output voltage. Therefore.

It is not possible to obtain a stable output voltage.

[Problem that the invention seeks to solve]

As is clear from the above explanation, conventional low voltage DC
The /DC converter has a problem in that the stability of the output voltage is poor.

Therefore, the present invention provides a DC/I that solves the above problems.
) C converter at low cost.

[Means for solving problems]

The present invention provides a direct current power supply with one primary winding (N
P) and the transistor collector emitter are connected in series, and the output current from the other primary winding (NB) of the transformer flows between the pace and the emitter of the transistor, and the In a self-excited DC/DC converter, self-excited oscillation is performed by causing the currents of
(Note) A circuit that uses a diode to take out the flyback output stored in the transformer while the Lancer is off to create a voltage with a polarity different from the output voltage.

It includes a circuit that adds the different polarity voltage and the output voltage, and a circuit that compares the divided voltage of the added voltage and a reference voltage, and controls the on/off time of the transistor according to the comparison result. It is characterized by stabilizing the output voltage.

〔Example〕 An embodiment of the present invention will be described with reference to FIG.

In this embodiment, a diode 1'D2 is added to the circuit shown in FIG.
It is characterized by the provision of a negative voltage setting circuit consisting of an i-capacitor C3, a resistor R3, a Zener diode zD2, and the like.

That is, a negative potential is taken out by a diode D2 connected to the negative winding NB, smoothed by a capacitor C3, and a negative constant voltage ve is obtained by a resistor R6 and a Zener diode ZD2. By selecting the primary winding N of the transformer T1, the Zener diode ZD2 can be selected to have the characteristics shown by the curve 32 in FIG. By doing this, the 0 potential and the negative potential °C become stable potentials. This negative potential °C and output voltage V. Detection resistors R, , R, are connected between ut and its partial voltage and Zener diode Z
Compare the Zener voltage of D1 and control the control transistor Tr2.

In this way, the negative potential °C becomes the output voltage V. By adding ut to ut, a Zener diode having the characteristic of curve 32 in FIG. 6 can be used.

Note that the negative potential taken out by the diode D2 and smoothed by the capacitor C3 is the output voltage V. Since the flyback voltage is approximately proportional to ut, when the output voltage vout is stabilized, the negative potential is also stabilized. Therefore, even without using the resistor R6 and the Zener diode ZD, a more stable output voltage Vout can be obtained than the conventional example shown in Figure 5. Figure 2 shows a diagram of the transformer T1 insulating the primary and secondary sides. This is another embodiment of the invention. Control transistor T
Using the output obtained through the diode D2, the pace current of r2 is applied to the photocazola phototransistor Tr3.
control. The phototransistor Tr3 is
Zener diode zD on the secondary side of transformer T□
3 and a photodiode PD1 controlled by a transistor Tr4. Then, the Zener diode ZD3, the transistor Tr4, and the photodiode PD1 transfer the DC output obtained from the diode D3 and the capacitor C4 to the Zener diode Zn.
Negative potential and output voltage V stabilized at 4. It is driven by the voltage added to ut to produce an output voltage V. Stabilize ut.

FIG. 3 shows a third embodiment of the present invention, which is a DC/I)C converter for obtaining a negative output voltage. Diode D2 and capacitor C3 create a flyback output positive potential °C, and output V. Add it to ut←).

A stable negative output voltage V compared to the Zener potential of the Zener diode ZD1. Get ut(-). In addition, in this embodiment, a resistor and a Zener diode (respectively, resistor R in Fig. 1) are used to further stabilize the positive potential.
5. Compatible with Zener diode ZD2) is omitted.

〔Effect of the invention〕

As described above, the self-excited DC/DC converter according to the present invention can obtain a stable low voltage with an inexpensive configuration, and its practicality is excellent.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of the first embodiment of the present invention, FIG.
Each figure shows the second part of the present invention. A circuit diagram of a third embodiment,
Fig. 4 is a basic circuit diagram of a well-known self-excited DC/DC converter, Fig. 5 is a circuit diagram of a conventional example that is an improved version of the circuit in Fig. 4, and Fig. 6 is a diagram showing the relationship between the Zener diode current and Zener voltage. Characteristic diagram showing the relationship. In the figure, T1 is a transformer, ZD, to ZD4 are Zener diodes, PD1 is a photodiode, and Tr5 is a phototransistor. Figure 5 Figure 6

Claims (1)

[Claims] 1. One primary winding (NP) of a transformer and the collector-emitter of a transistor are connected in series to a DC power source, and the output current from the other primary winding (NB) of the transformer is connected to the base-emitter of the transistor. The currents in the primary windings NP and NB are connected so as to flow between the emitters, and the currents in the primary windings NP and NB perform positive feedback to cause self-oscillation, and the secondary output of the transformer is rectified and smoothed using a diode and a capacitor. a self-excited DC/DC converter that obtains a direct current output by using a diode to take out the flyback output stored in the transformer while the transistor is off to create a voltage with a polarity different from the output voltage; comprising a circuit that adds the different polarity voltage and the output voltage, and a circuit that compares the divided voltage of the added voltage and a reference voltage, and controls the on/off time of the transistor according to the comparison result, A self-excited DC/DC converter, characterized in that the output voltage is stabilized.